The same person who gave you the interest amount paid over the phone should also be able to tell you what the mortgage principal is. You can go ahead and work on your return and come back to the mortgage section at a later date.
If you normally take the standard deduction instead of itemized deductions, you don't need to enter mortgage interest or any other itemized deduction. If there was no Schedule A included in your 2018 tax return, then you took the standard deduction in 2018.
